1. Field of the Invention
The present invention relates to transient voltage surge suppression systems, and in particular to a system and method for protecting transient-suppressing elements utilized in a transient voltage surge suppression system against over-voltage conditions.
2. Background Information
Electrical systems, such as an electrical power distribution system, periodically experience over-voltage conditions, such as transient over-voltage conditions, also called “surges.” Over-voltage conditions are problematic to electrical systems because they may cause damage to the loads, such as an electronic device or other hardware, that are coupled thereto. As a result, transient voltage surge suppression (TVSS) systems have been developed to protect the loads from over-voltages that would otherwise damage the loads. TVSS systems typically provide such protection by coupling various types of known transient-suppressing elements between the phase, neutral and/or ground conductors of an electrical power distribution system.
As is known in the art, transient-suppressing elements, such as metal-oxide varistors (MOVs), silicon avalanche diodes (SADs) and gas tubes, typically assume a high impedance state under normal operating voltages. When the voltage across a transient-suppressing element exceeds a pre-determined threshold rating, however, the impedance of the element drops dramatically, essentially short-circuiting the electrical conductors and “shunting” the current associated with the over-voltage through the transient-suppressing element and away from the load.
MOVs are probably the most commonly used transient-suppressing elements. An MOV consists of two plates separated by an insulator, such as a metal oxide, that has a known voltage breakdown characteristic. When the voltage between the two plates reaches a certain level (the voltage breakdown level), the insulator breaks down and conducts current. MOVs, however, have operational limitations that must be taken into account when designing a TVSS system. Specifically, all MOVs have a maximum transient current rating that, if exceeded, may cause the MOV to fail. An MOV may also fail if subjected to repeated operation, even if the maximum transient current rating is never exceeded. The number of repeated operations necessary to cause failure is a function of the magnitude of transient current conducted by the MOV during each operation: the lower the magnitude, the greater the number of operations necessary to cause failure.
In light of these limitations, prior art TVSS systems have been developed that use multiple MOVs in parallel combination such that the MOVs share the total transient current. Each individual MOV in such a configuration only conducts a portion of the total transient current, making it less likely that any individual MOV will exceed its maximum transient current capacity. In addition, a TVSS system that uses a plurality of parallel MOVs can withstand a greater number of operations because of the lower magnitude of transient current conducted by each individual MOV. Moreover, a parallel combination of MOVs is advantageous because the failure of any individual MOV will not cause a complete loss of TVSS system functionality.
When an MOV fails, due to exceeding its maximum current rating or due to frequent operation, it initially falls into a low impedance state in which it draws a large steady-state current from the electrical system. This current, if not interrupted, will drive the MOV into a thermal runaway condition, typically resulting in an explosive failure of the MOV and damage to or destruction of the TVSS system as a whole. To avoid the explosive failure of MOVs in a TVSS system, appropriately-rated current-limiting elements, such as a fuse, are typically employed in series with MOVs, preferably with one such current-limiting element being in series with each MOV. Prior art TVSS systems employing multiple MOVs and one or more fuses are described in, for example, U.S. Pat. No. 5,153,806 to Corey, U.S. Pat. No. 4,271,466 to Comstock, U.S. Pat. No. 6,636,409 to Kladar et al., and U.S. Pat. No. 6,678,140 to Jakwani et al.
The problem with using fuses to protect against MOV failure is that fuses, while effective in many conditions, are not reliable over the full range of fault currents that may occur. In particular, a fuse may open in response to certain over-current conditions (resulting from an over-voltage) that would not be a problem for (i.e., cause the failure of) the associated MOV. Such fuses are commonly referred to as “nuisance fuses” (having been opened under a condition that was not necessary to protect the MOV) and must be replaced, which is both expensive and inconvenient. Thus there is a need for a system for protecting transient-suppressing elements, such as MOVs, employed in a TVSS system from over-voltage conditions (and the over-currents that result therefrom) that is reliable over the full range of over-currents that may occur.